The rollout of DC Combo plugs has already started
Because the Model 3 will be equipped with a CCS Combo charging inlet, the Superchargers need to be retrofitted with DC Combo plugs.
The Superchargers will be equipped with two plugs - one for DC plug compatible with Type 2 inlet in Model S and Model X and a second one with DC combo. The first such stations are already up and running.
More about CCS expansion
Interestingly, Tesla left a sign on the Superchargers: "Model 3 priority", which probably is there to suggest that owners of Model S and Model X move to Superchargers not yet retrofitted, as there will not be enough stalls for all of the incoming Model 3.
The DC Combo plugs used by Tesla doesn't look pretty, but well that's what people will use around the world outside of North America and a few other countries.